Combustible gas sensors employ catalytic combustion to measure combustible gases or vapors in air up to the Lower Explosive Limit (LEL) of the gas. Known sensors include a matched pair of elements, typically referred to as a detector and compensator (reference element). The detector comprises a platinum wire coil embedded within a bead of catalytic material. The compensator is similar except that the bead does not contain catalytic material and as a consequence is inert. Both elements are normally operated in a Wheatstone bridge circuit, which will produce an output only if the resistance of the detector differs from that of the compensator. The bridge is supplied with a constant dc voltage that heats the elements to 500-550° C.
Combustible gases are oxidized only on the detector element, where the heat generated increases its resistance, producing a signal proportional to the concentration of combustible gas. The compensator helps to compensate for changes in ambient temperature, pressure, and humidity, which affect both elements equally.
A catalytic bead sensor is typically operated at 500 degrees Celsius with a power consumption on the order of 200-350 mW. The temperature of 500 C is chosen in order to be able to detect all combustible gases such as methane and still have a reliable response across temperature, humidity, and maintain linearity across the full range of the sensor. Portable instruments that employ a catalytic sensor have restricted run-times and size due to the power requirements of the sensor.
It would be desirable to provide a miniature combustible gas detector that runs weeks at a time before charging the battery. Preferably the power requirements of such sensors could be reduced while maintaining the robustness, linearity and resistance to poisoning as is expected from current sensors.